Axial movement linear gauging head

ABSTRACT

A linear gauging head, or cartridge head, comprises a casing (1), a spindle (10) movable with respect to the casing along a longitudinal axis, a feeler (12) coupled to a first end of the spindle and a differential position transducer with windings (13, 14, 15), fixed with respect to casing (1), and a core (16) coupled to a second end of spindle (10). The spindle is guided by recirculating ball bushings (18, 19) with rolling elements (29) that cooperate with the spindle and thus allow its displacement with respect to the casing.

TECHNICAL FIELD

The invention relates to a linear gauging head comprising support andprotection means with a casing defining a longitudinal geometrical axis,an elongate, substantially cylindrical-shaped element, axially movablewith respect to the support and protection means, a feeler elementcoupled to an end of the elongate element, a position transducer,including mutually displaceable parts connected to the casing and to theelongate element, respectively, thrust means located between the supportand protection means and the feeler element, and guide means, forguiding axial displacements of the elongate element with respect to thecasing, with at least an axial bearing including a plurality of rollingelements cooperating with the elongate element.

BACKGROUND ART

Axial movement linear gauges, or gauging heads, that have similarcharacteristics are from time conventional in the art. Two embodimentsare disclosed in U.S. Pat. No. 4,347,492. A first head, illustrated asprior art in FIG. 1 of the U.S. patent, has the typical structure of theaforementioned, so-called "cartridge" heads, including a cylindricalspindle, axially sliding within a casing by means of a guide deviceconsisting of an antifriction bearing and carrying at one end a feelerfor touching the workpiece to be checked and at the other end aferromagnetic core, that translates inside associated windings, as aconsequence of axial displacements of the spindle.

The antifriction bearing comprises balls and a cage with holes forseating the balls, that contact both the external surface of the spindleand the internal surface of the casing. The sliding of the cylindricalspindle is enabled by the rolling of the balls on the two surfaces withwhich it contacts. Consequently, slidings of the spindle cause slidingsof the whole cage.

The structures of the cartridge heads disclose in the U.S. patent aresubject to some drawbacks attributable to the great delicacy and carerequired for the assembly of the guide device.

In fact, for the purposes of a correct performance, the employedantifriction bearings have the need to undergo delicate assemblyoperations, owing to the fact that, among casing, balls and spindle,there need to be a coupling with a theoretically null clearance, with avery tight tolerance. Furthermore, it is necessary that the variouscomponent parts be in well defined reciprocal longitudinal positions,for the sake of preventing inappropriate limitations to the displacingof the cage of the bearing in the course of the slidings between spindleand casing. This means that the component parts have to be manufacturedso that one part takes account of the other and assembled with greatcare, hence implying additional expenses.

The "cartridge" heads with guide devices of this type are subject tofurther drawbacks, arising from undesired displacements of the cageseating the balls with respect to the previously mentioned accuratelongitudinal position. These displacements could be due, for example, tovibrations of the head and the unavoidable clearances that in practiceexist. These displacements more frequently occurring in thoseapplications where the measuring head has a vertically arrangedmeasurement axis can improperly limit the possibility of the bearingdisplacing and alter the possible reciprocal displacements betweenspindle and casing and, as a consequence, vary the measuring range, tilljamming the spindle.

Antifriction bearings including balls are used in a number of devicescomprising mutually movable elements, such as he probe for coordinatemeasuring machines that is shown in international patent applicationWO-A-8301301. The probe has a stylus carrying a tip and angularlymovable in any radial direction. Movements of the tip are mechanicallytransmitted to an external switch through a ball and cone to the probehousing by means of an antifriction bearing with balls.

British patent application GB-A-2107410 discloses a recirculatingball-spline assembly to be incorporated in industrial machines such asmachine tools and industrial robots and including a sleeve, a pair ofcage halves, eight sets of balls and a spline shaft having a squarecross-section.

DISCLOSURE OF THE INVENTION

Object of the present invention is to provide a cartridge head that hasa particularly simple structure and guarantees high standards ofaccuracy, repeatability, reliability and a reduction of costs and timerequired for the assembly of the various parts with respect to the knownstructures.

This is achieved by a gauging head wherein the formerly mentioned axialbearing further includes a hollow support element, fixed with respect tothe casing, and a guide structure housed in the hollow support element,the hollow support element and the guide structure defining internalrolling surfaces, said rolling elements being housed inside the supportelement and adapted to cooperate with the internal rolling surfaces forrecirculating in said hollow support element.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the linear gauging head according to theinvention is now described in more detail with reference to the enclosedsheets of drawings, given by way of non limiting example, wherein:

FIG. 1 is a longitudinal cross-sectional view of a gauging headaccording to an embodiment of the invention;

FIG. 2 is an enlarged scale cross-sectional view of the gauging headshown in FIG. 1, taken along line II--II in FIG. 1; and

FIG. 3 is a longitudinal cross-sectional view of a detail of the gauginghead shown in FIG. 1, taken along line III--III in FIG. 2.

BEST MODE FOR CARRYING OUT THE INVENTION

The axial movement gauging head shown in FIG. 1 comprises support andprotection means with a tubular, substantially cylindrical-shaped, steelcasing 1 that defines a longitudinal geometrical axis, a substantiallycylindrical internal surface 2 with a limiting annular projection 2' anda threaded end portion 6, and a rear closure element 3 coupled (morespecifically, glued) to casing 1. A spool 4 is housed in casing 1 anddefines a longitudinal opening 5 and three external annular seats 7, 8and 9.

An elongate element, or spindle, 10 is inserted in and movable withrespect to casing 1 and a feeler element with a spherical feeler 12, forcontacting a workpiece to be checked 48, is coupled (for example,screwed) to an end of spindle 10 that protrudes from the tubular casing1.

An inductive differential position transducer comprises a primarywinding 13, two secondary windings 14 and 15 and a core 16 made offerromagnetic material. The windings 13, 14 and 15 are tightlywound--according to a known technique--about spool 4 at the annularseats 7, 8 and 9, respectively, whereas core 16 is fixed (for example,glued) to a stem 17 that is coupled to spindle 10 at the opposite endwith respect to the one carrying the feeler element 12. Spool 4 withassociated windings 13, 14 and 15 is coupled to tubular casing 1, i.e.the external surface of windings 13, 14 and 15 is glued to the internalsurface 2 of tubular casing 1.

Guide means, for guiding axial displacements of spindle 10 with respectto casing 1, comprise two axial bearings, with recirculating rollingelements, for example recirculating ball bushings, 18 and 19, per seknown, housed in casing 1 at longitudinally reciprocally spaced outpositions. In the bearings 18 and 19, the rolling elements, inparticular balls 29, roll on inside surfaces and contact the externalsurface of spindle 10, as will be better explained hereinafter. As alsoshown in FIG. 2, each of the recirculating ball bushings 18 and 19 has ahollow support element 20 (and 20') with a substantially cylindricalshape, that defines an internal surface 49 and houses a guide structure50 made, for example, of synthetic resin polymers, such as "Teflon"(registered trade mark), and fixed to the hollow support element 20 incontact with the aforesaid surface 49. The guide structure 50 defines asubstantially cylindrical longitudinal through hole 21 for the partialinsertion of spindle 10. Through hole 21 defines a cylindrical surface22 of the guide structure 50 that has longitudinal slits 28. The guidestructure 50 has moulded portions 11, that, together with portions ofthe surface 49, define internal rolling surfaces 23 that limit internalcirculation tracks 24 where balls 29 are seated. Each track 24 has afirst longitudinal portion 25, connected with one of the slits 28, asecond longitudinal portion 26, substantially reciprocally parallel, andcurved connecting portions 27 between these longitudinal portions 25 and26. The arrangement of the moulded portions 11 is such that thelongitudinal portions 25 and 26 of each track 24 are adjacent to eachother and to the cylindrical surface 22. The slits 28, have specificdimensions so as to withhold the balls 29 within the associated rollingtrack 24 and enable the balls 29 to partially protrude with respect tothe cylindrical surface 22 and touch the external surface of spindle 10when they are in the associated first longitudinal portion 25.

Elements for the longitudinal positioning and clamping comprise a first,tubular-shaped spacer element 30 and a second, tubular-shaped spacerelement 31 housed in casing 1 and arranged, respectively, between theannular projection 2' and the support element 20 of bearing 18, andbetween the support elements 20 and 20' of the two bearings 18 and 19and a threaded clamping ring nut 32, also tubular-shaped, coupled to thethreaded end portion 6 of casing 1 with an end abutting on a basesurface of the hollow element 20' of bearing 19.

A pin 33 is radially coupled to spindle 10 and carries at its free endan idle small wheel 34. A slit 35, longitudinally formed in the secondspacer element 31, houses--with limited angular clearance--the smallwheel 34 that slides therein in the course of the longitudinaldisplacement of spindle 10 with respect to casing 1, hence limiting therotation of spindle 10 about its axis. Thrust means comprise an abutmentring 37, coupled to and coaxial with spindle 10, and a compressionhelical spring 36 arranged between a base surface of the support element20' of bearing 19 and the abutment ring 37.

An additional abutment ring 38, coupled to spindle 10, is in abutment ona base surface of an adjustment ring nut 39, internally coupled to theclamping ring nut 32 by means of a threaded coupling, for defining andadjusting the longitudinal position of spindle 10, biased by spring 36,when feeler 12 does not contact the workpiece 48.

An abutment surface 41, integral with spool 4, and an abutment surface40 of spindle 10 cooperate for defining the stroke limit of spindle 10,when feeler 12 contacts workpiece 48, offering resistance to the bias ofspring 36. A flexible, tubular-shaped sealing gasket 44 has one of itsends coupled to the area where feeler 12 and spindle 10 are connected,and the other end coupled to the end portion 6 of tubular casing 1.

The rear, closure element 3 has a through hole 45 for the electricconnection of the windings 13, 14 and 15 of the differential transducerwith external power supply, display and processing devices(schematically shown and identified by reference number 46 in FIG. 1),by means of the wires of a cable 47.

The assembly of the various component parts of the described andillustrated gauging head is performed in a particularly simple and rapidway. More specifically, once spool 4--that carries the windings 13, 14and 15--has been coupled to casing 1 by gluing the windings as alreadydescribed, some component parts, namely abutment ring 38, bushing 19,pin 33, ring 37, the second spacer element 31 and spring 36, are coupledto spindle 10. Then, the first spacer element 30, bushing 18 and spindle10--carrying the formerly mentioned component parts--are inserted insequence in casing 1 and the whole is locked by ring nut 32. Theposition of the recirculating ball bushings 18 and 19, in particular thelongitudinal positions of the associated support elements 20 and 20' incasing 1 are so defined and fixed by means of the thrust of clampingring nut 32 and the presence of the spacer elements 30 and 31. When thegauging head is in normal operating conditions and there is no contactoccurring between feeler 12 and workpiece 48, as illustrated in FIG. 1,spring 36 urges spindle 10 to reach a rest position defined by thecooperation of the abutment ring 38 with the adjustment ring nut 39.Subsequently, when contact occurs (in any whatever manual or automaticknown way, herein neither illustrated nor described) between feeler 12and a surface of workpiece 48, spindle 10 displaces, with respect tocasing 1 and in opposition to the action of spring 36, guided by therecirculating ball bushings 18 and 19, along a rectilinear path parallelto the longitudinal axis of casing 1.

More specifically, balls 29 touch the external surface of spindle 10through slits 28 and roll at one side on it and at the other oncorresponding internal surfaces 23, due to the thrust that spindle 10undergoes, so circulating along tracks 24.

The displacement of spindle 10 causes core 16 to displace withinwindings 13, 14 and 15 and a corresponding output voltage variation atthe terminals of the secondary windings 14 and 15, according to theknown functioning principle of an inductive differential transducer. Bymeans of the electric connection comprising the wires of cable 47, thevoltage variation with respect to a zero condition (defined in a knownway in a previous zero setting phase of the head), is detected in theexternal devices 46 and a signal relating to the amount of displacementfrom the rest position is displayed.

The use of recirculating herein described and 19 in the herein describedand illustrated linear gauge provides specific characteristics insofaras simplicity, reliability and economic convenience are concerned. Theuse of recirculating ball bushings 18 and 19 enables, among otherthings, to assemble various component parts in an extremely simple way,as previously briefly described. This is due to the fact that, incontrast with what occurs in the known devices employing bearings thathave to displace too with respect to the spindle and the casing forenabling reciprocal displacements between spindle and casing, it is notnecessary to couple the various parts (casing, spindle, guide withballs) and define their reciprocal position with an extremely highdegree of accuracy and a theoretically null clearance. A limited radialclearance between bushings 18, 19 and spindle 10 does not give rise toproblems relating to undesired displacements, since the bushings 18 and19 are held in the correct position by the spacer elements 30, 31 andthe clamping ring nut 32. On the other hand, a limited clearance (forexample, in the order of one or two μm) can easily be achieved and doesnot affect the accuracy and repeatability characteristics of the head.According to other possible variants, there can be foreseen a gauge thatdiffers insofar as the material and/or shape and/or number of componentsare concerned with respect to what has been herein so far illustratedand described. There can be utilized just one recirculating ballbushing, with a greater longitudinal extension with respect to that ofthe disclosed bushings 18 and 19, or, on the contrary, there can beprovided more than two bushings.

Moreover, the reciprocal arrangement of the bushings 18 and 19 and spool4 that carries the windings 13, 14 and 15 and the coupling of core 16 tospindle 10 can differ (for example, bushings 18 and 19 can be arrangedat the sides of spool 4).

Furthermore, the axial bearings can comprise rolling elements other thanballs 29, as, for example, cylindric rollers or rollers of another knownshape.

I claim:
 1. A linear gauging head comprising:support and protectiondevices with a casing that defines a longitudinal geometrical axis, aspindle, axially movable with respect to the support and protectiondevices, a feeler element coupled to an end of the spindle, a positiontransducer including mutually displaceable parts connected to the casingand to the spindle, respectively, a thrust device arranged between thesupport and protection devices and the feeler element, and guide means,for guiding axial displacements of the spindle with respect to thecasing, with at least an axial bearing including a plurality of rollingelements cooperating with the spindle, wherein the axial bearing furtherincludes a hollow support element, fixed with respect to the casing, anda guide structure housed in the hollow support element, the hollowsupport element and the guide structure defining internal rollingsurfaces, said rolling elements being housed inside the hollow supportelement and cooperating with the internal rolling surface forrecirculating in said hollow support element.
 2. A linear gauging headaccording to claim 1, wherein said guide structure (50), defines mouldedportions (11), the hollow support element (20) and the moulded portions(11) defining said internal rolling surfaces (23).
 3. A linear gauginghead according to claim 2, wherein said rolling elements have aspherical shape, and the guide structure defines a longitudinal throughhole for the partial insertion of the spindle, the through hole defininga cylindrical surface that has longitudinal slits for enabling therolling elements to touch said spindle.
 4. A linear gauging headaccording to claim 3, wherein said internal rolling surfaces defineinternal circulation tracks for the rolling elements, said trackscomprising a first and a second longitudinal portion and two connectingportions between the longitudinal portions, said slits being located atthe first longitudinal portion of each internal circulation track.
 5. Alinear gauging head according to claim 4, wherein said secondlongitudinal portion of each circulation track is arranged adjacent tosaid first portion and the cylindrical surface of the hollow supportelement.
 6. A linear gauging head according to one of claims 1 to 5,wherein the guide means comprise at least a further axial bearing withan additional hollow support element, fixed with respect to the casing,the hollow support elements being arranged in mutually spaced outpositions in a longitudinal direction.
 7. A linear gauging headaccording to claim 6, wherein the support and protection means compriselongitudinal positioning and clamping elements (30,31,32) forcooperating with the casing (1) and the hollow support elements (20,20')for defining and fixing said positions of the hollow support elements(20,20') with respect to the casing (1).
 8. A linear gauging headaccording to claim 7, wherein said longitudinal positioning and clampingelements compriseat least a tubular-shaped spacer element, housed in thecasing and longitudinally arranged between the hollow support elements,and a threaded clamping ring nut, the casing defining a threaded endportion for cooperating with said threaded clamping ring nut.
 9. Alinear gauging head according to one of claims 1 to 5, wherein saidmutually displaceable parts of the position transducer comprise electricwindings, a spool, fixed with respect to casing for housing saidwindings and a core made of ferromagnetic material and coupled to thespindle, the spool defining a longitudinal opening, coaxial with respectto the casing, whereinto said core is arranged and is displaceable withrespect to the windings.
 10. A linear gauging head according to one ofclaims 1 to 5, wherein said thrust devices comprise an abutment ringcoupled to and coaxial with the spindle, and a compression helicalspring arranged between said abutment ring and the hollow supportelement of said axial bearing.